WeakHashMap

Hash table based implementation of the Map interface, with
weak keys.
An entry in a WeakHashMap will automatically be removed when
its key is no longer in ordinary use. More precisely, the presence of a
mapping for a given key will not prevent the key from being discarded by the
garbage collector, that is, made finalizable, finalized, and then reclaimed.
When a key has been discarded its entry is effectively removed from the map,
so this class behaves somewhat differently from other Map
implementations.

Both null values and the null key are supported. This class has
performance characteristics similar to those of the HashMap
class, and has the same efficiency parameters of initial capacity
and load factor.

Like most collection classes, this class is not synchronized.
A synchronized WeakHashMap may be constructed using the
Collections.synchronizedMap
method.

This class is intended primarily for use with key objects whose
equals methods test for object identity using the
== operator. Once such a key is discarded it can never be
recreated, so it is impossible to do a lookup of that key in a
WeakHashMap at some later time and be surprised that its entry
has been removed. This class will work perfectly well with key objects
whose equals methods are not based upon object identity, such
as String instances. With such recreatable key objects,
however, the automatic removal of WeakHashMap entries whose
keys have been discarded may prove to be confusing.

The behavior of the WeakHashMap class depends in part upon
the actions of the garbage collector, so several familiar (though not
required) Map invariants do not hold for this class. Because
the garbage collector may discard keys at any time, a
WeakHashMap may behave as though an unknown thread is silently
removing entries. In particular, even if you synchronize on a
WeakHashMap instance and invoke none of its mutator methods, it
is possible for the size method to return smaller values over
time, for the isEmpty method to return false and
then true, for the containsKey method to return
true and later false for a given key, for the
get method to return a value for a given key but later return
null, for the put method to return
null and the remove method to return
false for a key that previously appeared to be in the map, and
for successive examinations of the key set, the value collection, and
the entry set to yield successively smaller numbers of elements.

Each key object in a WeakHashMap is stored indirectly as
the referent of a weak reference. Therefore a key will automatically be
removed only after the weak references to it, both inside and outside of the
map, have been cleared by the garbage collector.

Implementation note: The value objects in a
WeakHashMap are held by ordinary strong references. Thus care
should be taken to ensure that value objects do not strongly refer to their
own keys, either directly or indirectly, since that will prevent the keys
from being discarded. Note that a value object may refer indirectly to its
key via the WeakHashMap itself; that is, a value object may
strongly refer to some other key object whose associated value object, in
turn, strongly refers to the key of the first value object. If the values
in the map do not rely on the map holding strong references to them, one way
to deal with this is to wrap values themselves within
WeakReferences before
inserting, as in: m.put(key, new WeakReference(value)),
and then unwrapping upon each get.

The iterators returned by the iterator method of the collections
returned by all of this class's "collection view methods" are
fail-fast: if the map is structurally modified at any time after the
iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.

Causes the current thread to wait until another thread invokes the
notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.

WeakHashMap

Constructs a new WeakHashMap with the same mappings as the
specified map. The WeakHashMap is created with the default
load factor (0.75) and an initial capacity sufficient to hold the
mappings in the specified map.

entrySet

Returns a Set view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation, or through the
setValue operation on a map entry returned by the
iterator) the results of the iteration are undefined. The set
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll and
clear operations. It does not support the
add or addAll operations.

forEach

Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception. Unless
otherwise specified by the implementing class, actions are performed in
the order of entry set iteration (if an iteration order is specified.)
Exceptions thrown by the action are relayed to the caller.

Parameters

action

BiConsumer: The action to be performed for each entry

get

Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key
k to a value v such that (key==null ? k==null :
key.equals(k)), then this method returns v; otherwise
it returns null. (There can be at most one such mapping.)

A return value of null does not necessarily
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to null.
The containsKey operation may be used to
distinguish these two cases.

Parameters

key

Object: the key whose associated value is to be returned

Returns

V

the value to which the specified key is mapped, or
null if this map contains no mapping for the key

isEmpty

Returns true if this map contains no key-value mappings.
This result is a snapshot, and may not reflect unprocessed
entries that will be removed before next attempted access
because they are no longer referenced.

Returns

boolean

true if this map contains no key-value mappings

keySet

Returns a Set view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation), the results of
the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations. It does not support the add or addAll
operations.

remove

Removes the mapping for a key from this weak hash map if it is present.
More formally, if this map contains a mapping from key k to
value v such that (key==null ? k==null :
key.equals(k)), that mapping is removed. (The map can contain
at most one such mapping.)

Returns the value to which this map previously associated the key,
or null if the map contained no mapping for the key. A
return value of null does not necessarily indicate
that the map contained no mapping for the key; it's also possible
that the map explicitly mapped the key to null.

The map will not contain a mapping for the specified key once the
call returns.

Parameters

key

Object: key whose mapping is to be removed from the map

Returns

V

the previous value associated with key, or
null if there was no mapping for key

replaceAll

Replaces each entry's value with the result of invoking the given
function on that entry until all entries have been processed or the
function throws an exception. Exceptions thrown by the function are
relayed to the caller.

Parameters

function

BiFunction: the function to apply to each entry

size

Returns the number of key-value mappings in this map.
This result is a snapshot, and may not reflect unprocessed
entries that will be removed before next attempted access
because they are no longer referenced.

Returns

int

the number of key-value mappings in this map

values

Returns a Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own remove operation),
the results of the iteration are undefined. The collection
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll and clear operations. It does not
support the add or addAll operations.